Add multi select paint and fill to GridMap

[jamesmintram]

Part of godotengine/godot-proposals#10992

This PR adds support for multi select in the GridMap editor, doing so has a few effects:

• Painting - a random tile from the selection will be used
• Fill - area filled with random tiles from selection
• Clicking - repeatedly clicking the same cell will choose another tile randomly
• Existing functionality should remain unaffected

In addition to the new functionality I carried out some tidying around naming. I found it hard to follow mesh_palette_library and mesh_palette given one variable name is a subset of the other. I renamed them:

mesh_library and mesh_palette

I also made naming more explicit throughout as it was easy to get mixed up between various "library index" vs "palette index" vs "library id" as they are all ints.

The selection behaviour has been generalized around a "collected" of selected tiles from the palette, which also supports a single item and zero items being selected (ie maintains existing behaviour)

[jamesmintram]

@Nodragem this is reading for review/testing

cc'ing @ryevdokimov as you approved previous commits in this area

[Nodragem]

Sounds great! can't wait to have a look at it.

@jamesmintram I had a quick look, it seems that the feature is working as intended on my side, very nice!

However, there are, historically, actions that revert the multi-selection to 1 item being selected. While these "side-effects" were not important before, it can become frustrating now that multi-selection is being used.

I noticed for now that interacting with the below buttons/widgets are cancelling the multi-selection:

Would it be easy for you to make sure that multi-selection is preserved after interaction with this widgets/buttons?

[jamesmintram]

Sure, thanks for checking that out. I'll get that fixed up today